Coolant condition sensor apparatus

ABSTRACT

A dual function sensor particularly useful with vehicular coolant systems indicates when a coolant liquid becomes corrosive to such cooling system materials as well as when the liquid falls to a low level condition. A reference and a sense electrode are used to probe the condition of the coolant liquid. Integral electronics provide signal conditioning and transmitting to indicate both corrosive and low level coolant conditions. The sensor assembly mounts directly onto a tubular coupling on the vehicle radiator by pushing the assembly onto the coupling until a spring wire element snaps past a lip formed on the free distal end of the coupling. An electrical connector shroud extends from the assembly and accommodates a mating male connector which is pushed onto the shroud until a clip mounted on the male connector snaps over a locking tab located on the shroud. The male connector typically is connected to an engine control module (ECM).

CROSS REFERENCES TO RELATED APPLICATIONS

Application Ser. No. 780,467 filed of even date herewith containsrelated subject matter to that of this application.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates generally to apparatus for monitoring selectedconditions of liquid, such as engine coolant in a vehicular radiator,and more particularly to a sensor which can be used to provide remoteindication of the degree of corrosivity of the coolant, the presence orabsence of the coolant at a selected level in the radiator or bothconditions as desired.

2. Description of the Prior Art

Vehicular cooling systems are composed of several components whichinclude the radiator, circulating pump, passages in the engine block andassociated tubing. Since the system is composed of metallic parts, thereis a need to prevent, or at least mitigate, corrosion in order toprolong the useful life of the system. To this end it has become commonpractice to add chemical substances to the coolant liquid which serve toinhibit corrosion of the metal surfaces which come in contact with theliquid. Such substances are known as inhibitors and generally form afilm on the metal surfaces thereby protecting them from corrosion. Thus,commercially available permanent antifreeze includes an inhibitor whichis effective in preventing corrosion, however, over the course of timethe corrosion inhibiting characteristic of the liquid can become lesseffective due to various factors.

At the present time the use of aluminum for cylinder heads and coolingsystems in automobiles is becoming more common. Aluminum cylinder headsrun hotter than iron cylinder heads. This combined with aluminum'shigher susceptibility to corrosion leads to the potential for aphenomena known as "hot transport corrosion." This is a process wherebyaluminum corrosion products are transported from the hottest areas,typically in the head, to the coolest, that is, the radiator. Thesecontaminents interfere with good heat transfer and degrade the heatrejection capability of the system. Aluminum heads are also lesstolerant to overheat conditions than iron and run the risk of warping atelevated temperatures which can result from a low coolant condition.Also, the aluminum used in radiators and heater cores is moresusceptible to corrosion than the traditional copper-brass system whichmakes proper maintenance more critical to prevent failure due toperforation.

In U.S. Pat. No. 4,147,596, assigned to the assignee of the presentinvention, a system is disclosed and claimed in which a potentialmeasuring circuit employing at least two electrochemical electrodescomposed of dissimilar metals is located so as to be immersed and incontact with coolant liquid. The electrodes are mounted in a tubularhousing which is provided with a threaded portion so that it can bescrewed into a threaded bore of a wall confining the coolant liquid.When the inhibiting characteristic is effective, a first range ofelectrical potential exists between the electrodes; however, when theinhibiting characteristic becomes ineffective for any reason, a secondrange of electrical potential exists therebetween. Upon reaching athreshold level, as the potential moves into the second range,indicating means are actuated to provide a suitable indication of thecondition.

In U.S. Pat. No. 4,253,064, also assigned to the assignee of the presentinvention, another condition of the coolant is sensed utilizing the sameelectrodes which are part of a coolant inhibition characteristiccondition sensing system. In that system the presence or absence ofliquid is determined by making use of the electrical resistance betweenthe two electrodes without causing any appreciable current through theliquid which current would adversely effect the ability to sense thecoolant inhibition characteristic condition. The resistance between theelectrodes serves as part of a feedback network in a feedbackoscillator. The feedback oscillator incorporates a dc measuringamplifier for measuring the electrode potential as an indication of thecorrosion inhibition characteristic of liquid in which the electrodesare immersed. The same amplifier provides the amplification necessaryfor oscillation when the resistance between the electrodes increases toa threshold level indicating an absence of liquid. The feedback networkincludes a capacitor which not only determines the frequency ofoscillation, along with appropriate resistance, but also blocks anypossible DC current which might otherwise flow through the electrodes.The sensing of liquid presence or absence is accomplished between afirst electrode and ground while the second electrode may be directlygrounded, coupled to ground through a large capacitor or resistor orseparated from the first electrode by an electrically insulative barrierand thus coupled to ground through the liquid clinging to the insulatorseparating the second electrode from ground.

However the use of the feedback network in both the coolant inhibitioncharacteristic sensing function and the level sensing function tends tolimit the flexibility of the system. For example, adjusting thethreshold sensing parameters of one function sometimes may affect theother function. The use of the feedback network in both functions alsosubjects the system to some degree of noise sensitivity which couldcause nuisance tripping under certain conditions. Another limitation ofthe prior art involves the fact that the electrochemical sensing ofcorrosivity requires the sensing of negative potentials. In dealing withthis the prior art provided a negative power supply which rendered thesystem more complex and costlier than desirable. Another limitation wasthe output format of the system (DC output for corrosivity and AC outputfor low level). This made sensor output difficult to decode.

It is an object of this invention to sense electro-chemical potentialsof electrodes in a liquid, including potentials below ground, as anindication of corrosivity of the liquid as well as sensing the presenceor absence of the liquid using a common set of electrodes. Anotherobject is the provision of a system which can be used to sense eitherthe function of corrosion inhibition effectiveness of a liquid or thefunction of the presence or absence of such liquid or both functions ifdesired. Yet another object is the provision of a sensor used for eitheror both functions which is readily mounted on a vehicular cooling systemwhich is inexpensive yet reliable and long lasting. Another object isthe provision of a sensing system for sensing the functions of liquidcorrosivity and liquid level for which the threshold values for thefunction are readily and independently calibratable and one which hasselected hysteresis which is also calibratable. It is another object ofthe invention to provide an engine coolant condition sensor assemblywhich is inexpensive to manufacture and is easily installed in a liquidreservoir in a tamper proof manner. Another object is the provision ofsuch a sensor assembly which houses not only the sensing elements butalso electronics used to process or condition the electrical signalgenerated by the sensing elements and transmit it to a remote location,such as the instrument panel of an automobile, to give visual or audioindication of the condition of the coolant.

Other objects, advantages and details of the apparatus provided by theinvention appear in the following detailed description of the preferredembodiment of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a top plan view of a sensor assembly made in accordance withthe invention;

FIG. 2 is a front elevation of the FIG. 1 assembly;

FIG. 3 is a bottom view of the FIG. 1 assembly;

FIG. 4 is an side view of the FIG. 1 assembly;

FIG. 5 is a cross sectional view taken on line 5--5 of FIG. 2 with theaddition of a tubular coupling of an automotive radiator on which thesensor assembly is mounted and with the circuit components on circuitboard 32 and the encasing potting material not shown;

FIGS. 6 and 6a are enlarged elevation views, front and siderespectively, of a bimetal sense electrode;

FIG. 7 is an enlarged view in cross section of a sensor assembly takenon line 7--7 of FIG. 1 with the circuit components on circuit board 32and the encasing potting material not shown;

FIG. 8 is a block diagram of custom integrated circuits used in thesensor assembly to process and transmit the electrical signals generatedby the electrodes;

FIG. 9 is a schematic diagram showing a potential level shifting portionof the input/conditioner integrated circuit of FIG. 8;

FIGS. 10-10c is a schematic diagram showing the processing integratedcircuit; and

FIG. 11 is a flowchart showing an algorithm which can be used incontrolling the energization of the warning lights indicating a lowcoolant liquid level and a failure of corrosion inhibition.

Dimensions of the parts shown in the drawings may have been modified forpurposes of clarity of illustration.

SUMMARY OF THE INVENTION

Briefly, in accordance with the invention a sensor assembly housing hasa first generally tubular compartment open at one end which is adaptedto be telescopically received in a tubular coupling of a vehicularradiator. A pair of spaced "O" rings provide az liquid tight sealbetween the tubular compartment and the coupling. First and secondelectrodes, elongated elements each having a generally U-shapedconfiguration in cross section taken perpendicular to its longitudinalaxis, are disposed in the tubular compartment with a fixed end attachedto a rivet which extends through an "O" ring seal into a generallyparallelepiped configured second compartment. The second compartmentcontains a circuit board mounting electronics used to condition theelectrical signals received by the electrodes and transmit them to aremote location such as an engine control module and the dashboard ofthe vehicle. The electronics are potted in the second compartment toprovide an effective environmental seal. A connector shroud extends fromone side of the second compartment to permit attachment to anappropriate wire harness.

According to a feature of the invention, the housing has side wall meansformed with grooves adapted to receive a spring wire locking element.The locking element has first opposed portions adapted to be received inthe grooves and second opposed portions which are adapted to be receivedunder a lip formed on the outer distal end of a tubular coupling of theradiator. In mounting the sensor assembly to a radiator, the springlocking element is placed with the first opposed portions received inthe grooves of the housing walls. The second opposed portions of thewire project inwardly so that at their at rest position they are spacedfrom one another a distance which is less than the outer diameter of thetubular coupling. The housing is merely pushed onto the tubular couplingwith the second opposed portions being cammed away from each other untilthe outer lip is passed at which point the second opposed portions ofthe wire spring back toward each other under the lip to securely lockthe housing onto the coupling. In order to remove the housing the freeends of the wire are pried apart to move the second opposed portions ofthe wire away from each other a distance greater than the outer diameterof the lip.

According to another feature of the invention the electronics comprisetwo integrated circuits, the first containing two PMOS devices whichallow sensing of negative potentials and also provide isolation betweenthe electrodes and between the electrodes and ground. These devicesshift a negative potential to a positive potential which is outputted toa bipolar integrated circuit.

According to a feature of the invention the operating threshold forcorrosion inhibition sensing is adjustable by changing either a currentreference resistor or a bias resistor which are separately connectableto the bipolar integrated circuit. The hysteresis of the corrosivitysensing is adjustable by connecting a resistor between two designatedpins of the bipolar integrated circuit.

According to yet another feature of the invention the level sensingportion uses the impedance between the two electrodes and is adjustableover a wide range of resistance threshold by varying aresistor-capacitor leg that connects to the bipolar integrated circuit.A capacitor is used as a noise filter to improve the switchingcharacteristics of the sensor while two diodes are used to provide a 1.4volt band of hysteresis.

According to a feature of the invention the output of the low level andcorrosive circuits contained within the bipolar integrated circuit arecombined into a logic network that results in a low level indicationoverriding a corrosivity warning.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring now to the drawings numeral 10 indicates a sensor assemblymade in accordance with the invention. Assembly 10 includes a housing 12preferably molded of suitable electrically insulative material such asZytel, a trademark of E. I. duPont de Nemours & Co. for glass fillednylon, having a first generally tubular compartment 14. The side wall oftubular compartment 14 is provided with two circular grooves 16, 18spaced from one another along the longitudinal axis of the tubularcompartment. "O" rings 20, 22 are disposed in the respective grooves toprovide a fluid tight seal between the compartment 14 and a tubularcoupling 24, shown in FIG. 5, in which the compartment is received.

With particular reference to FIGS. 3 and 5-7, first and secondelectrodes 26, 28 are disposed within compartment 14, each having a baseplate 26.3, 28.3, welded to a rivet, 26.1 and 28.1 respectively, whichextends through a respective aperture in wall member 14.1 for connectionwith electronics to be described below. "O" rings 26.2 and 28.2 arereceived around the rivets and are disposed in grooves formed in wallmember 14.1 in communication with the respective apertures to provide afluid tight seal. Suitable connectors, such as connector 26.5interconnect the rivets with the electronics as well as capture "O"rings 26.2, 28.2 in their respective grooves (see FIG. 7 which shows "O"ring 26.2 captured by connector 26.5).

The outer portion of tubular compartment 14 is preferably formed with ataper 14.2 to facilitate the placement of the compartment into tubularcoupling 24.

Housing 12 has a second compartment 30, generally configured as aparallelepiped, to accommodate electronics used to condition the signalsreceived from the electrodes and transmit the conditioned signals to aremote location such as the dashboard of a vehicle. The electronics aremounted on a circuit board 32 as will be described infra and are pottedtherein by any suitable electrically insulative potting material toprovide an effective environmental seal.

A connector shroud 34 extends laterally from one of the side walls ofcompartment 30 and encloses spaced terminals GND, VOUT and VCC whichextend through the wall of the compartment from circuit board 32. Asuitable wiring harness (not shown) is receivable in the shroud forconnection to the terminal. Preferably a locking tab 34.1 is provided onshroud 34 to lock the harness in its position of connection with theterminals.

Electrode 26 is a reference electrode and is preferably formed of silverbut could also be formed of other standard commercial reference materialsuch as the calomel family, silver alloys including Ag/Ag halide andAg/AgO, the copper family such as Cu/Cu halide and Cu/CuSO₄ or otherstable reference material.

Electrode 28 is a sense electrode and is preferably formed of a cladelement having a first layer 28.6 of steel bonded to a second layer 28.8of aluminum. These materials are especially adapted for use with coolingsystems which embody aluminum components, as taught in U.S. Pat. No.4,147,596 mentioned supra. If the sensing system of the presentinvention is used in other environments, then other sense electrodematerials could be used depending on the materials used for thecomponents to be monitored.

Electrodes 26, 28 are elongated elements extending in a directiongenerally parallel to the longitudinal axis of the tubular compartment14 and are formed with a base plate 26.3, 28.3 bent at an angle ofapproximately 90° to the remainder of the electrode. The base plate iswelded or otherwise suitably fixed and electrically connected to a rivetas described above. The remainder of the length of the electrode isgenerally U-shaped with respect to a cross section taken perpendicularto the longitudinal axis of compartment 14, the side walls of theU-shape providing mechanical strength to the electrode.

Housing 12 is also provided with opposed side wall members 36, 38depending downwardly from wall 14.1 with grooves 36.1, 38.1 formed atopposite ends of each wall member. A spring wire locking element 40 isreceived in grooves 36.1, 38.1 and is used to lock the assembly to acoupling member such as the tubular coupling 24 of a vehicular radiator.Element 40, best seen in FIG. 3, may be formed of a length of stainlesssteel or other suitable wire material and is bent into a generallyrectangular configuration with opposite ends 40.1 spaced slightly apartin the middle of one of the legs of the rectangle. Opposed legs 40.2 and40.3 are generally straight while opposed legs 40.4 and 40.5 are bentinwardly so that opposed portions 40.6 and 40.7 in respective legs 40.4and 40.5 are at a distance from one another which is less than the outerdiameter of an outwardly extending lip 24.2 formed on the free distalend of tubular coupling 24 (see FIG. 5). As best seen in FIG. 3 housing12 has side walls forming an outer peripheral surface which extends inall lateral directions beyond element 40.

The assembly 10 is normally provided to the vehicle manufacturer withlock element 40 already disposed in grooves 36.1, 38.1 so that all thatneed be done in installing the assembly is to push it onto tubularcoupling 24. As the coupling engages the spring element 40, it pushesportions 40.6 and 40.7 further apart until element 40 passes beyond thelip at which point the portions 40.6 and 40.7 spring back toward oneanother and against the outer wall of tubular coupling 24 below lip24.2. In this regard, it is preferred that lip 24.2 be formed with ataper 24.3 (FIG. 5) to facilitate the outward camming action of portion40.6 and 40.7 as the assembly is installed.

With the assembly mounted in its locked position, there is no danger ofit becoming accidentally dislodged or loose, as would be the case if aconventional threaded connection were used. Further, the need forprotecting threads of a conventional connection from damage prior toinstallation is obviated and the time required for installationsubstantially reduced compared to connection techniques using threadedmembers.

If it is desired to remove the assembly, ends 40.1 are pushed apartuntil portions 40.6 and 40.7 are spaced from one another a distancegreater than the diameter of lip 24.2 at which point the assembly can beslipped off the coupling.

The circuitry used in practicing the invention will be described withparticular reference to FIG. 8 showing a block diagram of the electroniccomponents mounted on circuit board 32 and FIGS. 9 and 10 showingschematically such circuitry. The circuit utilizes two surface mounted,custom integrated circuits, the first a CMOS chips SN 28880 (referred tobelow as '80) and the second a bipolar chip SN 28881 (referred to belowas '81). As seen in FIG. 8 V_(IN) and R_(IN) represent the coolantliquid in which electrodes 26 and 28 are immersed. As the corrosioninhibition characteristic of the fluid varies, the potential differencegenerated between the two varies. The R_(IN) varies based on whether theelectrodes are immersed or not, that is, when inundated there may be, byway of example, an R_(IN) resistance on the order of one hundred ohmsbut when the level of the coolant falls below the electrodes exposingthem the R_(IN) increases beyond ten thousand ohms.

RO' is a one megohm resistor connected between electrode 28 and V_(CM)(V common mode) which is in turn shown connected to ground. V_(CM) andRO' comprise a representation of the electrode potentials andresistances with respect to engine ground.

The '80 device is constructed using CMOS technology in order to providelow input leakage, high input impedance to obviate the possibility ofelectrode loading as well as to allow sensing of potentials a volt belowthe negative supply rail, which in the present case, is ground, andcomprises PMOS transistors Q1' and Q2'. The input from electrode 28through pin VCR- of the '80 device is connected to the gate of Q1' andthe input from eletrode 26 through pin VCR+ is connected to the gate ofQ2'.

Each input of the '80 integrated circuit is preferably provided withelectrostatic discharge (ESD) protection comprising diodes D2-D10-fieldeffect transistors FET 1-FET 6 network to protect the circuit componentsfrom both positive and negative voltage spikes. As seen in FIG. 9,networks comprising devices D1-D10, RP 1, 2 and T1-T10 provide ESDprotection and bias to the circuit inpus of the '80 circuit and insurevery low current leakage into the electrode probe inputs at elevatedtemperatures. Leakage greater than 100 nano ampers over an extendedperiod of time would degrade the electrochemical potential developed bythe electrodes.

Transistors Q1' and Q2' level shift the potentials of the circuit inputsVCR- and VCR+ from the electrodes from a low as much as one volt belowground up to a level above ground with their outputs taken from the FETsources and supplied to VCRO- and VCRO+ pins which are connected to the'81 integrated circuit.

Thus the '80 IC allows the '81 bipolar IC to see potentials within itscommon mode range (above ground) while providing little or no inputleakage current which would adversely effect sensor potentials.

Turning back to FIG. 8, resistor R_(TH) connected between pin VCRO- ofthe '80 device and VCRI- of the '81 device sets the threshold at whichthe system works as will be referred to in more detail below. In orderto negate the offset error introduced by the use of R_(TH) causingdifferent current flow between the two transistors Q1' and Q2', resistorR_(DN) (drain) is connected between the drain of transistor Q2' andground to balance the current flow through the transistors.

Capacitor C1' coupled between pin VCR- and ground is an AC decouplingcapacitor which allows use of the same electrodes for sensing of bothlevel and corrosion inhibition. Capacitor C2' connected betweenelectrode 26 and pin VLV+ of the '81 device is also part of thedecoupling network.

Pin VCRO- of the '80 device is connected to pin VCRI- of the '81 devicethrough resistor R_(TH) while pin VCRO+ is connected directly to pinVCRI+ of the '81 device. As mentioned above, the potentials seen at pinsVCRI- and VCRI+ are shifted up from the potentials seen at theelectrodes so that both potentials are positive.

Resistor R1' is serially connected to capacitor C3' and both areconnected between pin VLV- of the '81 device and ground. Changing thevalues of these components will change the threshold value of the levelsensing function as will be described in more detail below.

A resistor R_(BIAS) is connected between pin BIAS of the '81 device andground. This resistor determines the amount of current which passesthrough resistor R_(TH). A change in the value of resistor R_(BIAS) willtherefore change the current reference. The current reference alsoprovides positive hysteresis. For example, if the device were triggeredat 300 mv, it would not turn off until a selected lower value isreached, for example 200 mv. In the preferred embodiment the device isinitially set at a hysteresis value of 100 mv by connecting the HYSI pinto the BIAS pin. This value can be changed by connecting a resistor (notshown) between the HYSX pin and the BIAS pin. These options areschematically shown by the switches connected to pin HYSI and HYSX andinterconnected by dotted lines on FIG. 8.

The dashed line block labeled coolant condition sensor is essentially ahigh impedance differential amplifier Q3' that receives the sense andreference potentials which have been level shifted by the PMOStransistors.

The level sensing portion comprises differential amplifiers Q4', Q5',and buffer Q6'. Capacitor C4' connected between pin VLVO and groundserves as a time delay to filter out any noise.

Resistor RH20 is connected between the supply voltage VCC and thepositive input to amplifier Q4' while resistor RH21 is connected betweenthat input and ground. Resistors RH22 and RH23 are respectivelyconnected between the output of amplifier Q4' and its negative andpositive inputs.

The output of amplifier Q4' is fed to the input of dual comparator Q5'.Diodes Q73 and Q74 are connected across the positive and negative inputsto comparator Q5' which in turn are connected through respectiveresistors RH24', RH25' to VCC and ground. Thus the diodes provideapproximately a ±0.7 v comparator threshold range. That is, if there isa low level condition with the electrodes out of the liquid, amplifierQ4' begins to oscillate between ground and the positive supply rail. Ifthere is any noise, the ±0.7 volts serves as a buffer so that the inputto amplifier Q5' must go that much above or below in order to flip backand forth.

The output from amplifier Q5' is fed to hysteresis buffer Q6' which thenis fed to a logic block composed of nor gates. The logic ensures thatthe corrosivity function is overridden by the level function so that alow coolant level condition takes precedence over the sensing of thecorrosion inhibition characteristic of the coolant.

The output drive block indicated by dashed lines provides high currentswitching on the order of tens of milliamps. The buffers shown in effectrepresent two switches to ground. With a low level condition the twoshut off so that VOUT pulls to VCC. When a corrosive condition occurs,the upper switch turns on which pulls the output to ground. During anormal condition the lower switch turns on to ground resulting in avoltage divider comprising R7' and R8' and giving half of the value ofVCC.

With particular reference to FIG. 10 showing schematically the '81device, the dashed line section CCS (coolant condition sensor) BIASidentified as A, comprises transistors Q70-72 and Q56-63 which serve ascurrent sources to insure proper circuit bias in several devicetransistors.

Dashed line (Section B) comprises the Coolant Condition Comparator (Q3'of FIG. 8). The inputs VCRI- and VCRI+ are outputs from the '80 device.Transistors Q1-5 form a standard differential amplifier whose outputfeeds both the CCS Hysteresis (Section C) as well as the Output Logic(Section E). Current sources, transistors Q54, Q55 of the BandgapReference (Section D) connected as current mirrors supply bias currentto the bases of transistors Q1 and Q2. The current sourced bytransistors Q54 and Q55 whose bases are connected to the collector oftransistor Q6 is determined by the resistor value connected to the BIASpin 3 which is in series with emitter of transistor Q6.

Current supplied by transistor Q55 is also injected across the thresholdresistor R_(TH). The resulting voltage drop across this resistordetermines the comparator threshold voltage. A potential at the VCRI+input that is greater than the voltage drop across resistor R_(TH) willturn off transistor Q5. This turns on transistor Q31 in the Output Logic(Section E) which allows current to bias transistors Q37 and Q38 on. Apotential below the threshold value results in turning on transistor Q32which in turn sinks bias current away from Q37. This results in shuttingoff Q37 which turns off output transistor Q38. Due to the externalresistors R7', R8' (see FIG. 8) connected to VOUT1/VOUT and VOUT2 acorrosive indication results in a VOUT below 1 volt, a non-corrosiveindication of approximately 2.5 volts and a low level indication above 4volts. The output of the comparator section also feeds into the CCSHysteresis (Section C).

The CCS Hysteresis (Section C) provides an adjustable amount ofhysteresis for the coolant condition comparator and comprisestransistors Q49-50, 64 and 65 and diode Q51. Input pins HYSI, HYSX andBIAS are used 20 to set up the integrated circuit for either of twomodes: internal or external hysteresis. The internal mode uses aconnection between HYSI and BIAS. During a corrosive conditiontransistor Q5 is off which results in transistor Q49 turning on and Q50off. Transistor 25 Q71 can then push bias current through diode Q51which in turn flows through pin HYSI and through the R_(BIAS) resistorconnected to ground at the BIAS pin. A decrease in the current throughtransistor Q6 occurs which in turn decreases the current flow through 30transistors Q54 and Q55. This results in a decreased voltage drop acrossthe threshold resistor R_(TH).

A noncorrosive condition causes transistor Q5 to turn on resulting in nocurrent flow through Q51. This increases current through transistors Q6,Q54, and Q55.

The HYSX pin is used for external hysteresis. A resistor connectedbetween pins HYSX and BIAS allows an adjustable hysteresis value. Thecurrent flow or absence of flow through Q65, R18 and the externalresistors sets the hysteresis thresholds. Transistor Q64 biases theemitter of transistor Q65 at VCC/2.

The Bandgap Reference Section D of the circuit provides a well regulatedreference which is utilized by other portions of the circuit andcomprises transistors Q7-Q11 and Q58-Q63. A nominal 1.235 volt referenceis supplied at the emitter of transistor Q6 (R_(BIAS) pin). Resistor R15matches resistor R_(BIAS) while transistor Q6 and Q7 VBE's are alsomatched. Transistors Q52 and Q53 are current mirrors used to minimizethe bias current offset errors for transistors Q6, Q54 and Q55. DiodeQ12 and Q13 are used in conjunction with transistor Q69 and Q66 toprovide a two VBE potential allowing the bandgap to power up properly onapplication of power to the circuit. The current flow through Q54 andQ55 is a function of the bandgap voltage divided by the resistance atthe BIAS or HYSX pins.

The Output Logic (Section E) uses inputs from the Coolant Level BufferWith Hysteresis (Section I) and the Coolant Condition Comparator(Section B) portions of the circuit. These inputs cause the Output LogicCircuit, (Sections F and J) to properly drive the output driver circuitto provide theappropriate output to the analog instrumentation input.The logic is configured to allow a low level indication to override botha noncorrosive and a corrosive indication. A corrosive coolant resultsin transistors Q5 being off, Q31, on the Q32 off. This in turn allowsQ70 current to bias transistors Q37 and Q38 on. With the resistorconfiguration on the output as described supra, this results in a sensorVOUT of less than one volt.

A noncorrosive condition causes transistor Q5 to turn on, transistor Q31turns off and Q32 turns on, and bias current is taken away from theDriver #1 (Section F). Transistor Q34 turns off in addition to Q31,therefore providing bias current to Driver #2 (Section J). The resultingindication is one half of VCC. If there is a low level condition,transistors Q48 turns off which results in a turn on of both Q35 an Q33.This results in a turn off of Output Drivers #1 and #2 and a VOUTindication above 4 volts (assuming a 5 volt power supply).

The output pin VOUT1 of Driver #1 (Section F) is connected to tworesistors (see FIG. 8), one to VCC, R8' and the other to R7'. The otherend of R7' is connected to VOUT2. Pin VOUT1 is tied directly to theoutput VOUT. Resistors RH32 and RH34 are bias resistors and RE35provides short circuit protection. Transistor Q36 turns on if the dropacross resistor RE35 is above the transistor VBE. The turning on of Q36limits the output current and protects Q38 from an overload condition.

The output in VOUT2 of Driver #2 (Section J) is connected to resistorR7' (FIG. 8) which is in turn is connected to input VOUT1. ResistorsRH33 and RH36 are bias resistors and RE37 provides short circuitprotection with transistor Q39 functioning in a manner similar totransistor Q36 of section F.

The coolant level OP-AMP (Section G) comprises an input stage composedof transistors Q14-Q20, Q79 and Q80. Transistor Q69 is a current sourceused to provide a proper bias at the VLV+ pin input through resistorsRH20 and RH21. Resistors RH22 and RH23 go from the output of the OP-AMPback to the inputs setting up the unstable oscillator front end.Capacitor C2 serves to improve the stability of the OP-AMP.

The resistor and capacitor R1'/C3' connected to the VLV- input (FIG. 8)form one leg of the unstable oscillator input. If the impedance of theother leg is greater, the OP-AMP breaks into oscillation. Otherwise asteady stateoutput voltage results.

The output of the OP-AMP feeds directly into the inputs of twodifferential input pairs Q22, Q23 of the coolant level comparator(Section H). The remaining two inputs are taken from the resistor/diodedivider leg formed by RH24, Q73, Q74 and RH25. This leg provides thethreshold for these two differential amplifiers (in this case acting ascomparators).

With no coolant level OP-AMP oscillation Q28 is turned off which resultsin a discharged capacitor which is connected to the VLVO pin.Oscillation causes Q28 to turn on which results in a charged capacitorand a turning off of transistor Q48.

During an adequate coolant level condition transistor Q28 is off and Q43is on and Q47, Q46 and Q78 of Coolant Level Buffer with Hysteresis(Section I) are all turned on. Q67 and Q68 provide two VBE hysteresiswith Q67/Q68 turning on or off dependent upon the state of the coolantlevel comparator. The capacitor provides a filtering time constant forthe signal from the comparator. When transistor Q43 is turned on, Q48 ison and when Q43 is off so is Q48.

FIG. 11 shows a flowchart of an algorithm which can be used inconjunction with the engine control module (ECM) shown in dashed linesin FIG. 8 to obtain the desired performance including early warning tothe driver of the vehicle of a low coolant condition which can lead toengine overheating and damage as well as early warning of a corrosivecoolant condition which can lead to engine or cooling system failure. Aselected number of samples, e.g. six, or low level indications will turnon an "add coolant" light in the dashboard of the vehicle (not shown).The light is turned off by a selected number of samples, e.g. six, of anormal coolant condition. The "change coolant" light (not shown) isturned on after a selected number of corrosive coolant samples (eg.six). This light remains on for the remainder of the driving cycle,regardless of the add coolant indication.

Electrical performance characteristics of a system built in accordancewith the invention are shown in Table I as follows:

                  TABLE I                                                         ______________________________________                                                               TYPI-                                                  CHARACTERISTICS                                                                              MIN     CAL     MAX    UNIT                                    ______________________________________                                        Supply Voltage (VCC)   5.0            VOLTS                                   Supply Current (ICC)            5     MA                                      Corrosivity Threshold                                                                        200             400    MV                                      (V CAL)                                                                       Level Threshold (R LEV)                                                                      0.5             10.0   K OHMS                                  "Corrosive" Output             1.0    VDC                                     (VOUT)                                                                        "Noncorrosive" Output                                                                        2.0     2.5     3.0    VDC                                     (VOUT)                                                                        "Low Level" Output                                                                           4.0                    VDC                                     (VOUT)                                                                        Operating Temperature                                                                        -40             +125   DEG C                                   Range                                                                         ______________________________________                                    

This system incorporated components shown in Table II as follows:

                  TABLE II                                                        ______________________________________                                        IC SN 28880   (14 pin)                                                        IC SN 28881   (8 pin)                                                         C1', C4', C5' .1 μf monolithic                                                           ceramic chip capacitor                                          C3'           .0047 μf monolithic                                                        ceramic chip capacitor                                          C2'           .01 μf monolithic                                                          ceramic chip capacitor                                          R7'           300 ohm thick film chip resistor                                R8'           348 ohm thick film chip resistor                                R1'           1.0 Kohm thick film chip resistor                               R.sub.TH, R.sub.DN                                                                          select resistor-                                                PCB 32        printed circuit board                                           R.sub.BIAS    6.1 Kohm thick film chip resistor                               ______________________________________                                    

Sensor systems made in accordance with the invention can of course beemployed with liquids other than automotive coolants such as heattransfer liquids used in connection with energy systems or machine toolsor the like.

In the foregoing specification, the invention has been described withreference to a specific exemplary embodiment thereof. However, it willbe evident that various modifications and changes may be made thereuntowithout departing from the scope of the invention as set forth in theappended claims. The specification and drawings are accordingly, to beregarded in an illustrative rather than in a restrictive sense.

What is claimed is:
 1. A replaceable liquid condition sensor assemblyhaving electrode probe means adapted to be immersed in a liquid forproviding an electrical signal representative of a selected conditionand having electronic means for processing the signal for transmissionto a remote location and terminal means for detachably connecting theassembly to provide a power source to the assembly and an output pathfor the processed signal characterized in that the assembly comprises ahousing having side walls forming an outer peripheral surface and havinga first compartment, signal processing electronics disposed in thecompartment, terminal means extending from within the compartment inelectrical connection with the electronics to a point outside thecompartment, shroud means extending from the housing around the terminalmeans extending from the housing, and potting material sealing theelectronics in the first compartment, the housing having integral withthe first compartment a second open ended compartment generally in theform of a hollow tube depending from the housing, the electrode probemeans extending from the first compartment through apertures into thesecond compartment, electrode sealing means disposed in the apertures toprevent liquid from seeping from the second compartment to the firstcompartment, sealing means disposed about the hollow tube to seal amating coupling in a liquid reservoir defining means and a releasable,movable locking element to releasably lock the assembly to the matingcoupling comprising an elongated spring wire bent back on itself andhaving first opposed portions which are movable toward and away fromeach other, the housing formed with opposed wall means depending fromthe housing in generally the same direction as the hollow tube and onopposite sides thereof, the walls having locking element receivinggrooves, the locking element received in the locking element receivinggrooves with the first opposed portions of the locking elementprojecting into the space between the opposed wall means and the hollowtube, the peripheral surface of the housing extending in all lateraldirections beyond the locking element, the assembly receivable on thecoupling with the coupling extending into the space between the opposedwall means and the hollow tube, the locking element being generallyrectangular shaped in plan view with the first opposed portionscomprising first and second oppositely disposed legs, each having aninwardly extending bowed portion, the first and second legs beingjointed at a first end thereof by a third leg, a second end of the firstand second legs attached to a discontinued fourth leg having two freeend portions extending toward but spaced from each other, the two freeend portions being disposed closely adjacent to a portion of the wallmeans to thereby minimize tampering and accidental dislodgement of thesensor assembly from the mating coupling.
 2. A replaceable liquidcondition sensor assembly according to claim 1 further characterized inthat the hollow tube of the second compartment has a distal free end anda taper is formed on the outer surface of the distal free end tofacilitate insertion of the assembly into a mating coupling.
 3. Areplaceable liquid condition sensor assembly according to claim 1further characterized in that the electrode probe means comprise a firstelongated generally channel shaped electrode of a reference material anda spaced second elongated generally channel shaped electrode of twolayers of different metal materials.
 4. An automotive cooling systemhaving radiator means defining a reservoir and having a tubular couplingprojecting therefrom, a replaceable liquid condition sensor assemblyhaving electrode probe means adapted to be immersed in a liquidcontained in the reservoir for providing an electrical signalrepresentative of a selected condition and having electronic means forprocessing the signal for transmission to a remote location and terminalmeans for detachably connecting the assembly to provide a power sourceto the assembly and an output path for the processed signalcharacterized in that the assembly comprises a housing having side wallsforming an outer peripheral surface and having a first compartment,signal processing electronics disposed in the compartment, terminalmeans extending from within the compartment in electrical connectionwith the electronics to a point outside the compartment, shroud meansextending from the housing around the terminal means extending from thehousing, and potting material sealing the electronics in the firstcompartment, the housing having a second open ended compartmentgenerally in the form of a hollow tube depending from the housing andsized to fit closely into the tubular coupling projecting from theradiator means, the electrode probe means extending from the firstcompartment through apertures into the second compartment, electrodesealing means disposed in the apertures to prevent liquid from seepingfrom the second compartment to the first compartment, sealing meansdisposed about the hollow tube to seal the hollow tube to the couplingand a releasable, movable locking element to releaseably lock theassembly to the tubular coupling comprising an elongated spring wirebent back on itself and having first opposed portions which are movabletoward and away from each other, the housing formed with opposed wallmeans depending from the housing in generally the same direction as thehollow tube and on opposite sides thereof, the walls having lockingelement receiving grooves, the locking element received in the lockingelement receiving grooves with the first opposed portion of the lockingelement projecting into the space between the opposed wall means and thehollow tube, the peripheral surface of the housing extending in alllateral directions beyond the locking element, the assembly received onthe coupling with the coupling extending into the space between theopposed wall means and the hollow tube, the first opposed portions ofthe locking element comprising first and second legs joined at a firstend thereof by a third leg, a second end of the first and second legsattached to a discontinued fourth leg having two free end portionsextending toward but spaced from each other, the two end portions beingdisposed closely adjacent to a portion of the wall means to therebyminimize tampering and accidental dislodgement of the sensor assemblyfrom the tubular coupling.
 5. An automotive cooling system according toclaim 4 further characterized in that the hollow tube of the secondcompartment has a distal free end and a taper is formed on the outersurface of the distal free end to facilitate insertion of the assemblyinto the tubular coupling projecting from the radiator.
 6. An automotivecooling system according to claim 4 further characterized in that thecoupling has a distal free end formed with a radially outwardlyextending lip, the locking element receiving grooves so spaced from thehousing that as the assembly is pushed on to the coupling, the lockingelement is biased apart by the lip until the lip is disposed between thegrooves and the housing whereby the locking element moves inwardly underthe lip to lock the assembly to the coupling.
 7. An automotive coolingsystem according to claim 6 further characterized in that the lip isformed with a tapered surface to facilitate camming of the lockingelement as the assembly is pushed onto the radiator coupling.
 8. Anautomotive cooling system according to claim 6 further characterized inthat the locking element is generally rectangular shaped in the planview with the first opposed portions comprising first and secondoppositely disposed legs, each having an inwardly extending bowedportion.
 9. An automotive cooling system according to claim 4 furthercharacterized in that the electrode probe means comprise a firstelongated generally channel shaped electrode of a reference material anda spaced second elongated generally channel shaped electrode of twolayers of different metal materials.